The present invention relates generally to a magnetic resonance imaging (xe2x80x9cMRIxe2x80x9d).
Magnetic resonance imaging is widely used in medicine for providing images of internal structures within the body of a patient. MRI offers numerous advantages over other imaging techniques such as x-ray and computerized axial tomography (CAT) imaging. MRI does not expose the patient to ioning radiation and can capture images of tissues which are not readily shown by other techniques.
To produce an MRI image, a strong, uniform magnetic field is applied to the region of the patient to be imaged. Radio frequency (xe2x80x9cRFxe2x80x9d) energy is applied to this region of the patient by a transmitter and antenna. The RF energy excites atomic nuclei within the patient""s tissues. The excited nuclei spin at a rate dependent upon the magnetic field. As they spin, they emit faint RF signals, referred to herein as magnetic resonance signals. By applying small magnetic field gradients so that the magnitude of the magnetic field varies with location within the patient""s body, the magnetic resonance phenomenon can be limited to only a particular region or xe2x80x9cslicexe2x80x9d of the patient""s body, so that all of the magnetic resonance signals come from that slice. Moreover, by applying additional magnetic field gradients, the frequency and phase of the magnetic resonance signals from different locations within the slice can be made to vary in a predictable manner depending upon the position within the slice. Thus, it is possible to distinguish between signals from different parts of a slice.
If this process is repeated numerous times using different gradients, it is possible to derive a map showing the intensity or other characteristics of magnetic resonance signals from particular locations within the patient""s body. Because these characteristics vary with the concentration of different chemical substances and other chemical characteristics of the tissues, different tissues provide different magnetic resonance signal characteristics. When the map of magnetic resonance signal characteristics versus location is displayed in a visual format, such as on a computer screen or printed image, the map forms a picture of the structures within the patient""s body, with different tissues having different intensities or colors. The magnets used in MRI imaging must provide a magnetic field which is strong and uniform. Preferably, the magnet of the MRI imaging apparatus has a patient-receiving space capable of receiving the torso of a normal human being, and provides a field strength of at least 1 kilogauss and preferably at least about 3 kilogauss or more. This field desirably is uniform to within about 1 part in 107 or better in an imaging volume at least about 25 cm in diameter in the patient-receiving space.
The required fields can be generated by a so-called air-core solenoidal superconducting magnet. These magnets have coils positioned along a horizontal axis and a tubular central bore inside the coils. The patient is placed inside this tubular bore while he or she is being imaged. Although magnets of this type can provide acceptable images, they subject to the patient to an intensely claustrophobic experience for the duration of the imaging procedure. Moreover, these magnets inherently require that the imaging procedure be conducted with the patient in a horizontal orientation, lying prone or supine on a bed.
Other magnets utilize ferromagnetic frames. These frames typically include a bottom pole support plate, a set of columns extending upward from the bottom pole support plate and a top pole support plate supported by the columns. Ferromagnetic poles extend upwardly from the bottom pole support plate and downwardly from the top pole support plate so that the poles define the patient-receiving space of the magnet between them. A source of magnetic flux such as superconducting or resistive electromagnet coils encircling the poles or a mass of permanent magnet material is associated with the frame. Magnetic flux passes into the patient-receiving space through the face of one pole and passes out of the patient-receiving space through the face of the opposite pole. The flux returns through the pole support plates and columns.
Certain ferromagnetic frame magnets provide numerous advantages including high field strength and good field uniformity. However, these magnets typically also require that the patient be in a horizontal position.
As disclosed in certain embodiments of U.S. Pat. No. 6,023,165, the disclosure of which is hereby incorporated by reference herein, and in co-pending commonly assigned U.S. patent application Ser. No. 08/978,084, the disclosure of which is also incorporated by reference herein, it is sometimes desirable to acquire an MRI image while the patient is in a vertical or nearly vertical position, such as a standing position. Other positions intermediate between a vertical and horizontal position as, for example, a Trendlenberg or reverse Trendlenberg position can be employed. Certain magnets disclosed in the ""165 patent and in the 978,084 application have ferromagnetic frames with horizontal pole axes and can accommodate a patient in a horizontal, vertical or intermediate position.
However, despite these advances in the art, still further improvement would be desirable. In particular, it would be desirable to provide magnets which combine ease of patient entry and egress and a relatively non-claustrophobic patient experience. It would also be desirable to provide apparatus which combines these features with the ability to image essentially any location in the body, including the head and feet, while the patient is in either a vertical or horizontal position. It would be desirable to provide these features in a magnet which can be built at reasonable cost and which provides good field characteristics. Further, it would be desirable to provide these features in a magnet which can be mounted so as to isolate the magnet from mechanical vibrations, for example, vibrations of the earth caused by vehicular traffic. These goals, taken together, present a significant challenge.
One aspect of the invention provides MRI apparatus. The apparatus according to this aspect of the invention desirably includes a magnet frame having two vertically-extending side walls and a pair of ferromagnetic poles projecting inwardly along a polar axis from each said side wall towards the other said side wall. The apparatus also includes a top flux return structure extending between the side walls above the poles and a bottom flux return structure extending between said side walls the poles. The frame defines a patient receiving space within said interior space between the poles. Patient entry openings defined by the side walls and flux return structure desirably allow patient entry and exit. A floor structure preferably is provided in proximity to the bottom flux return structure so that a patient may enter said patient-receiving space by moving across the floor structure and entering into the frame on or above the bottom flux return structure. A source of magnetic flux such as electromagnet coils or permanent magnets provides magnetic flux in the patient-receiving space through said poles. The remainder of the frame provides a return path for the flux. The top flux return structure, bottom flux return structure or, preferably, both flux return structures, have openings aligned with said patient-receiving space. These openings preferably are of sufficient size to accommodate at least a part of the patient. An elevator may be provided for raising and lowering a patient relative to said frame. Thus, while a patient is disposed in the interior of the frame, the patient can be raised or lowered so that a part of the patient, such as the head or feet, protrudes into one of the openings in a flux return structure and another part of the patient is disposed in the patient receiving space, in alignment with the poles. Thus, essentially any part of the patient can be imaged while the patient is in a vertical orientation, such as in a standing posture.
In a particularly preferred arrangement, the apparatus includes a carriage, the elevator being mounted to the carriage for movement with the carriage into and out of the frame. The apparatus desirably further includes a patient support mounted to the elevator, the elevator being operative to raise and lower the patient support relative to the carriage and hence relative to the magnet frame. Most preferably, the elevator and the patient support can be pivoted relative to the carriage through a range of positions including a horizontal position and a vertical position, so that the patient can be positioned in essentially any orientation.
The ferromagnetic frame, with substantially symmetrical flux return elements, provides excellent field characteristics, but nonetheless allows patient positioning as discussed above. The frame can be supported by vibration isolators such as inflatable bladders.
A further aspect of the invention provides methods of magnetic resonance imaging. A method according to this aspect of the invention desirably includes the steps of positioning a patient within a magnet having a frame as discussed above. The positioning step desirably includes advancing the patient into the frame through an entry opening defined by the side walls and said flux return structures so that the patient passes into the frame above the bottom flux return structure and below the top flux return structure. The method further includes the step of conducting magnetic resonance imaging of the patient using the magnet. The step of positioning the patient may include orienting the patient substantially vertically within the frame and raising or lowering the patient relative to the frame so that one portion of the patient extends into an opening in one of said flux return structures and another portion of the patient is disposed in the patient-receiving space for imaging.
For example, the step of positioning the patient desirably includes advancing the patient into the frame on a support carried by a carriage, and the step of raising or lowering the patient preferably includes raising or lowering the support relative to the carriage. The step of advancing the patient into the frame may be performed while the patient is in a substantially horizontal orientation, in which case the step of positioning the patient further includes pivoting the support to bring the patient to the substantially vertical orientation while the patient is at least partially disposed within the frame of the magnet.
Methods according to this aspect of the invention provide extraordinary versatility in imaging.
These and other objects, features and advantages of the present invention will be more readily apparent from the detailed description of the preferred embodiments set forth below, taken in conjunction with the accompanying drawings.